This invention relates to an apparatus for controlling a residual heat removal system used for a nuclear reactor, which can safely cool the reactor within a short time after it is shut down.
To begin with, the outline of a typical nuclear power plant will be explained. A reactor is placed in a container vessel and the heat generated through nuclear reactions in the reactor core turns the coolant water into steam.
The steam is taken out through a main stream conduit externally of the container to drive a main turbine. The main turbine in turn drives a generator coupled directly to the main turbine so that electric power is generated. The steam, after having driven the main turbine, is condensed through a condenser into water. The condensed water is fed back as coolant water through a condensate pump and a feedwater pump into a reactor vessel. Thus, a nuclear power plant includes a closed loop of coolant water.
When a fault occurs inside or outside the reactor, several control rods are immediately inserted into the reactor, that is, the reactor is scrammed (the nuclear chain reaction in the fuel is stopped). Simultaneously with scramming, isolation valves provided in the main steam conduit immediately inside and outside the container vessel are closed, depending on the nature of fault, to prevent radioactive materials from escaping from the reactor vessel. With the isolation valves closed, the reactor core can no longer be cooled by the main cooling system including the main turbine. Since a reactor continues to generate decay heat for more than ten hours after scram, an auxiliary cooling system must be provided for the reactor which takes out the residual heat from the reactor to cool the reactor down when the isolation valves have been closed. Such an auxiliary cooling system is a residual heat removal system (hereinafter referred to simply as RHR system) with which the invention is concerned.
The RHR system comprises a closed loop which is started upon the closure of the isolation valves, the reactor steam is first led to a heat exchanger to be condensed into water and the condensed water is fed as coolant into the reactor vessel by a pump. This pump is so designed as to be driven by a turbine actuated by the reactor steam since in an emergency there is a large possibility that the power source in the plant is out of use and therefore the RHR system must operate without resorting to the plant power source. The steam, having done work on the turbine, is exhausted into another vessel. A make-up water reservoir communicates with the part of the pipes between the heat exchanger and the feedwater pump so as to prevent cavitation in the pump when the flow of water out of the heat exchanger decreases. Most of the above mentioned components of the RHR system are housed in the container vessel and the control of the system is performed by three separately provided control devices as follows.
(a) Device for controlling the flow of the condensed water:
This control device operates the heat exchanger inlet valve to control the internal pressure P.sub.H of the heat exchanger to a preset value (P.sub.H).sub.ref in order that the flow W.sub.H of the condensed water through the exchanger per unit time may be stabilized.
(b) Device for controlling the flow of feed water:
This device controls the aperture of the steam inlet valve of the turbine to control the flow W.sub.F of feed water to a preset value (W.sub.F).sub.ref in order that the coolant water may be fed into the reactor vessel at a constant rate.
(c) Device for controlling the flow of make-up water:
This serves to supply make-up water to the pump when the reactor-side pressure P.sub.S at the junction of the feedwater pipe and the make-up water pipe falls below a preset value (P.sub.S).sub.ref. Normally, (P.sub.S).sub.ref &gt;P.sub.S and the flow of make-up water W.sub.C =0 so that W.sub.H =W.sub.F.
According to this RHR system, since the flow W.sub.TB of water resulting from steam which has driven the turbine is exhausted, the level L.sub.R of the reactor water falls as measured by the expression (1) given below. ##EQU1## Here, L.sub.RO indicates the initial reactor water level, A.sub.R the cross sectional area of the reactor vessel, t the time for which the reactor is being operated and V.sub.f the specific volume of the reactor water, the reactor water meaning saturated water.
According to a test calculation for a boiling water reactor having a rated output of 1,100,000 KW, the reactor water level L.sub.R falls about two meters down in several hours after scram and thereafter the emergency core cooling system ECCS as a safety mechanism other than the RHR system starts operating so that a large amount of cooling water is injected into the reactor vessel to resume the initial level. This is a preferable in view of the safety of the reactor since the safety is double assured by the RHR system and the ECCS. However, the flood of the cooling water causes thermal impacts on the reactor core assembly and the reactor vessel, resulting in causing deterioration thereof. It is therefore preferable if the residual heat can be removed by the RHR system alone. The problems caused by the supercooling due to the operation of the ECCS arises mainly from the fall of the reactor water level due to the operation of the RHR system.
On the other hand, the RHR system must not only remove the residual heat but also observe the preset rate of the fall of, for example, the reactor water temperature to protect the members in the reactor vessel from thermal impacts. The following expression (2) denotes the rate of the fall of the reactor water temperature, i.e.-.DELTA.Temp/.DELTA.t, as one of the main temperature fall rate, calculated from the energy balance for a reactor with its ECCS out of operation. ##EQU2## In the expression (2), Q.sub.R indicates the decay heat in the reactor, T.sub.emp the temperature of the reactor water, M.sub.f the weight of the entire reactor water, i.sub.g the enthalpy of steam, i.sub.f the enthalpy of the reactor water (saturated water), and i.sub.F the enthalpy of the cooling water. In this case, it is clear that W.sub.H (i.sub.g -i.sub.F)=W.sub.F (i.sub.g -i.sub.F), both quantities indicating an amount of heat required to turn feed water at the flow rate of W.sub.H =W.sub.F into steam, and that the quantity W.sub.TB (i.sub.g -i.sub.f) denotes an amount of heat required to turn the condensed water at the flow rate of W.sub.TB into steam.
For example, the rate of the fall of the reactor water temperature must be maintained at a present value, e.g. 55.degree. C./hour, to keep the residual thermal stress in, for example, the container vessel smaller than an allowable limit. However, since the decay heat Q.sub.R decreases with time, the temperature fall rate increases when W.sub.F is kept constant, as seen from the expression (2), until it exceeds 55.degree. C./hour. Accordingly, the operator must change the preset value (W.sub.F).sub.ref for the coolant water flow from time to time by manual setting in order that the temperature fall rate may not exceed the maximum allowable value 55.degree. C./hour. If (W.sub.F).sub.ref is so set as to correspond to 30.degree. C./hour whereby the rate may not exceed 55.degree. C./hour, the problem of supercooling can be solved, but too much time is required to remove the residual heat.